Double grid tube



Patented Apr. 12, 1932 UNITED- STATES PATENT OFFICE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION Oli" PENNSYLVANIA.

1 DOUBLE Gm TUBE- Application filed May112, 1927. Serial No. 190,665.

My invention relates to vacuum tubes and j to methods of constructing them. More particularly it relates to so-called screen-grid f tubes for use as radio amplifiers and detectors. One of the objects of my invention is .to produce a radio amplifier and detector tube which shall have aminimum tendency to act as a source of undesired oscillations. V

Another object of my invention is to pro- 9 duce a tube in which the effective capacity beternally to the tube.

A further object of my invention is to devise a method of producing a shielding coat ing upon the interior of a tube and of connecting it to any desired electrode.

Other objects of my invention will be apparent from the following specification, taken in connection with the drawings, wherein Figure 1 is a longitudinal section ofa tube,

embodying my invention,'in the condition it is in prior to the deposition of a shielding coating upon its interior walls;

Fig. 2 is a longitudinal section of a tube after the deposition of the aforesaid coating and represents the tube in its finished and operative state.

' fying tubes which may be used in series,-

Figs. 3 and 4 are sections taken on t he lines IIIIII and IV'IV of'Fig. 1 respectively.

Asis well known, theordinary three-electrode radio tube of the prior art has an appreciable electrostatic capacity between its anode and control-electrode, in consequence of which it isliable to produce'undesired oscillations. The undesired oscillations im.- pose a limitation upon the number of amplisince these oscillations, generated because of the internal capacity of the tube, interact to produce the disturbances of audible frequencies which constitute the howling which'is so troublesome in present-day radio receiving sets. I

It has been found that, if an additional ILLIAM J. KIMMELL, on T'RAFFORRVPENNSYLVANIA, ASSIGNOR 'rofwns'rmenousn grid electrode be interposed between.v

anode and the control-grid of radio tubes of the conventionaltype and be maintained at a definite potential relative to the cathode,

the capacitybetween the anode and, control electrode is greatly decreased, with the consequent diminution of the troublesome oscillations just described. However, it is found that there is a certain amount of residual capacity still left between the anode and control-electrode so that the abovementioned difficultiesarenot completely eliminated by the simple structure just described.

I- have discovered that, if the interior wall of'the radio tube in the vicinity of its operative electrodes be covered, with a conducting coating connectedto the cathode, or alternatively .to the additional. grid' 'just mentioned, practically the zentireresidual capacity between the anode and control-electrode is eliminated, with the consequent complete elimination of the trouble from .self-oscilla tion. 7 I have further found that, by bringing the inleadingwire to the controlgrid throughthe top ofthe tube-rather than. through they conventional press supporting the inleading wires at the lower end of the tube, and by int'erposing between the plate electrode and the upper control-electrode lead a metallic shield, the slight remnant of capacity between the anode and the control. grid is done away with. 1

Referrin tothe drawings,.l denotes a vacuum-tig t glass radiov tubez'of ordinary type. having, at its lower end, a press 2 through which passe's inleading wires to a cathode i of conventional structure near the axis of the tube. Surrounding the oathode4 is a'control-grid .5 which may be of ordinary type. exceptthat' its inleading wire. 7 6 preferably enters the tube throu h the top insteadof through the press v2. ing-the control-electrode, in turn, is a second or screen grid 7 which is supported on two vertical wires 8 having their upper ends fixed in an insulator'9and their lower ends supported in the press 2. ;The screen gridiis adapted to be connected to external circuits through one lead. 8.,passing througliythe P s,-

urround- As shown in Fig. 4:, this plate may be formed of two semi-circular halves not making direct contact to each other, but individually welded to the screen-grid.

In the course of manufacture of this tube,

the press 2 isfirst formed with the structure v comprising cathode l, control grid 5, screen grid 7, anchor 9, plate 11 and anode 1O supported thereon. The control grid lead 6 pro- 10. The flexible wire is, therefore, held under jects above the top of this electrode structure, but is, of course, not yet sealed into the enclosing tube. This entire structure is inserted through the flared lower end ofthe enclosing tube 1 and sealed in place therein, the control-grid lead 6 being inserted through the top of the tube 1 and sealed thereto at the same time.

At the time this electrode structure is assembled on the press, a thin flexible wire 12, preferably of tungsten, is welded either to one of the cathode leads 3 or to the lead 8 of the screen grid and is welded insuch a position that, upon straightening out, its free end will press against the tube wall. However, for the time being, the other end of the flexible wire is welded to a small piece of metallic magnesium 13 which is, in turn, welded to the outside of the anode structure of the flexible wire 12 which was held bound by the magnesium is freed and it snaps into i contact with the magnesium coating 14 which .1s ust then settling upon the tube walls. It

is conducive to the success of this operation that the heating of the magnesium take place i as rapidly as possible; in other words that a quantity of magnesium be vaporized to pro-' duce a thick and readily conductive coating on the entire interior of the tube in the neighborhood of its electrodes. By locating the magnesium tab in the position shown in Fig. l, the tube walls surrounding the electrode group will be heavily coated, but practically no coating will deposit where any of the inleading or support Wires are sealed into glass. This prevents the magnesium coating from forming a short-circuit between any of the various electrodes.

The tube constructed in the foregoing manner is intended to be connected for use in the following way. The cathode is supplied with current from any convenient alternating-current or direct-current source. A source of unidirectional electromotive force may be connected between the cathode circuit and the lead of the screen grid, thereby maintaining the latter at a different and uniform potential with reference to the cathode. The input electromotive force is connected between the control grid lead 6 and the cathode 4, and the output circuit is connected to, the leads from cathode 4 and anode 10 in any manner usualin the ordinary detect-or or amplifier circuit arrangements.

Because of the interposition of the screen grid between the anode and the control grid; itintercepts nearly all the lines of electric force which would otherwise tend to flow from the interior face of'the anode. to the control grid. Thus the electrostatic capacity between the interior face of the anode and control electrode is substantially zero.

The metallic coating formed upon the interior of the tube is connected in the embodiment here shown, by the flexible lead 12 to the cathode, and is, therefore, maintainedi always at the cathode potential. In consequence, any stray lines of electromotive force emanating from the exterior face of the anode 1O find a convenient terminus upon this coating and practically none of them terminate upon the control-grid 'lead.

Furthermore, the shield 11, which is maintained at the potential of the screen-grid 7, intercepts lines of electric force tending to pass from the anode to the control-grid lead 6. Hence, the electrostatic capacity between the anode and the control grid is reduced practically to zero. In other words variations in the potential of either of these electrodes have practically no effect on the other;

As a result of the construction above described, I have produced a radio tube having practically zero capacity between its anode and grid electrodes. The presence of the screen-grid 11 also reduces practically to Zero the conductance of the path within the tube from anode'to the cathode. As aconsequence, the flow of current through the output circuit including the anode is determined almost entirely by the value of the control-grid potential. 'As a result, there is no tendency for the tube to go into self-oscillation; neither is there any tendency for a feedback of energy to take place from the anode circuit to the control grid circuit. The

tube is, therefore, quite strictly a one-way transmitter of modulations. I

If, as is ordinarily the case, a portion of the cathodecircuit is grounded, the conducting deposit on the interior of the tube walls is likewise maintained at ground potential. The electrodes inside the tube are, therefore, almost completely screened from disturbances due to electrical efiects originating externally to the tube.

It will be evident that, instead of connecting the interior coating of the tube to the cathode, I might arrange the flexible wire to connect it to, one of the other electrodes, such as the screen-grid, for example.

It will furthermore be evident that, instead of coating the interior of the tube with a metallic deposit, I might attain a similar shielding effect by a conducting coating on the exterior of the tube connected in circuit in similar fashion. It will further be evident that, while I have described magnesium as the metal to form a fusible connection for the flexible wire 12, other readily fusible metals may be substituted therefor. In fact, magnesium is only one of the metals commonly vaporized in high-vacuum tubes and known in the art under the name of getters. Any of these well known getters might beused for this purpose.

I have specified that the flexible wire shall be made of tungsten but it will be evident that materials other than tungsten might be arranged to spring or' even to fall by gravity to form a conducting bridge between th cathode and the conducting coating. 7

Instead of connecting the internal coating of the tube directly to one of the electrodes, I may leave it disconnected but cover a portion of the exterior of the tube with a conducting coating connected to the desired electrode-lead. The exterior and interior coatings then comprise a condenser and the induced charges thereonprodnce similar effects to those described above in'the case of conductive connection from cathode to coating as described above.

While I have described my method of shielding in connection a screen-grid tube, it will be evident that many of its desirable effects are equally applicable to electron-discharge tubes of other types. It is,

therefore, not limited in applicability to screen-grid tubes. that the screen-plate 11 and the coating on the interior of the tube are adjunctive features, and thateither'may be applied without the presence of the other. I I have, therefore, produced a tube which is almost completely free from the troublesome effects of electrode capacity and of disturbing influences originating in objects external to the tube. H

While I have described a particular embodiment of my invention,it will be evident It will further be evident having a pluralityof electrodes therein which comprises attaching a resilient conductor to one electrode under an elastic force tendmg to force said conductor into contact with the 'wall of the container, opposing said elastic force by a link of fusible material, exhausting said container and melting said fusible material. 7 p

. 2I'The method of producing an-electrical comprises attaching a resilient conductor to an electrode under an elastic force tending to "bridge the gap between said electrode and the container wall, opposing said elastic force by a link of vaporizablemetal, evacuating said container and vaporizing said metal.

'3. In combination, a vacuum-tight container holding an electrode group comprising a cathode, a-main electrode and a screen-electrode supported from one end thereof, a con- .trol-electrode, a support therefor from the other end of the container, a conducting shield on thewalls of said container substantially enveloping said electrode group, a connection from said coating to the cathode and a shield between" said control-electrode support and said anode and connected to said screen-electrode.

4. In combination, a vacuum-tight container holding an electrode group comprising a cathode, a main electrode, a screen-electrode and a control-electrode, a conducting coating .onthe interior of said container in the region of said electrode group and a resilient conductor connecting said coating to said cathode. 5. The method of constructing a vacuum 1 discharge device in a vacuum-tight container having a plurality of electrodes therein which tube having an anode member, a cathode memher, a conductive coating on the inside of said vacuum tube and having said coating connected to one of saidmembers whichcomprises formin said coating and connecting said cathode member to said coating by vapor- 'izing a getter.

In testimony whereof, I have hereunto subscribed my name this 10th day of May, 1927,

WILLIAM TKIMM' LL. 

